Disciplinary after sick absence

My partner was recently ill with a viral infection , which entailed a stay in hospital and regular blood tests to ascertaion the progress of recovery from the infection. The doctors treating her issued the required paperwork for her to send to her employer and she kept in regular contact with the person in charge of sick absence who was at the time very sympathetic expressing the opinion that she should remain off work until fully recovered. Upon return to work she has been informed that she is required to attend a disciplinary hearing purely because of her absence due to being sick with what was potentially an infectious illness. My questions are :

1) Is this procedure usual ?

2) In what way is this a discipline issue ( if anything maybe a capability investigation)?

3) What is the best way to argue against any disciplinary action?

I thank in advance all who contribute an answer